feat: each without as (#14396)

* feat: each without as

WIP

closes #8348

* properly

* docs

* changeset

* real world demo

* simplify

* typo

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/sixty-zoos-enjoy.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: support `#each` without `as`

documentation/docs/03-template-syntax/03-each.md

@@ -74,6 +74,30 @@ You can freely use destructuring and rest patterns in each blocks.
 {/each}
 ```
 
+## Each blocks without an item
+
+```svelte
+<!--- copy: false  --->
+{#each expression}...{/each}
+```
+
+```svelte
+<!--- copy: false  --->
+{#each expression, index}...{/each}
+```
+
+In case you just want to render something `n` times, you can omit the `as` part ([demo](/playground/untitled#H4sIAAAAAAAAE3WR0W7CMAxFf8XKNAk0WsSeUEaRpn3Guoc0MbQiJFHiMlDVf18SOrZJ48259_jaVgZmxBEZZ28thgCNFV6xBdt1GgPj7wOji0t2EqI-wa_OleGEmpLWiID_6dIaQkMxhm1UdwKpRQhVzWSaVORJNdvWpqbhAYVsYQCNZk8thzWMC_DCHMZk3wPSThNQ088I3mghD9UwSwHwlLE5PMIzVFUFq3G7WUZ2OyUvU3JOuZU332wCXTRmtPy1NgzXZtUFp8WFw9536uWqpbIgPEaDsJBW90cTOHh0KGi2XsBq5-cT6-3nPauxXqHnsHJnCFZ3CvJVkyuCQ0mFF9TZyCQ162WGvteLKfG197Y3iv_pz_fmS68Hxt8iPBPj5HscP8YvCNX7uhYCAAA=)):
+
+```svelte
+<div class="chess-board">
+	{#each { length: 8 }, rank}
+		{#each { length: 8 }, file}
+			<div class:black={(rank + file) % 2 === 1}></div>
+		{/each}
+	{/each}
+</div>
+```
+
 ## Else blocks
 
 ```svelte

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -1,4 +1,4 @@
-/** @import { ArrowFunctionExpression, Expression, Identifier } from 'estree' */
+/** @import { ArrowFunctionExpression, Expression, Identifier, Pattern } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { Parser } from '../index.js' */
 import read_pattern from '../read/context.js';
@@ -142,16 +142,25 @@ function open(parser) {
 				parser.index = end;
 			}
 		}
-		parser.eat('as', true);
+
+		/** @type {Pattern | null} */
+		let context = null;
+		let index;
+		let key;
+
+		if (parser.eat('as')) {
 			parser.require_whitespace();
 
-		const context = read_pattern(parser);
+			context = read_pattern(parser);
+		} else {
+			// {#each Array.from({ length: 10 }), i} is read as a sequence expression,
+			// which is set back above - we now gotta reset the index as a consequence
+			// to properly read the , i part
+			parser.index = /** @type {number} */ (expression.end);
+		}
 
 		parser.allow_whitespace();
 
-		let index;
-		let key;
-
 		if (parser.eat(',')) {
 			parser.allow_whitespace();
 			index = parser.read_identifier();

packages/svelte/src/compiler/phases/2-analyze/visitors/EachBlock.js

@@ -16,7 +16,7 @@ export function EachBlock(node, context) {
 	validate_block_not_empty(node.fallback, context);
 
 	const id = node.context;
-	if (id.type === 'Identifier' && (id.name === '$state' || id.name === '$derived')) {
+	if (id?.type === 'Identifier' && (id.name === '$state' || id.name === '$derived')) {
 		// TODO weird that this is necessary
 		e.state_invalid_placement(node, id.name);
 	}

packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js

@@ -47,8 +47,8 @@ export function EachBlock(node, context) {
 
 	const key_is_item =
 		node.key?.type === 'Identifier' &&
-		node.context.type === 'Identifier' &&
-		node.context.name === node.key.name;
+		node.context?.type === 'Identifier' &&
+		node.context?.name === node.key.name;
 
 	// if the each block expression references a store subscription, we need
 	// to use mutable stores internally
@@ -147,7 +147,7 @@ export function EachBlock(node, context) {
 	// which needs a reference to the index
 	const index =
 		each_node_meta.contains_group_binding || !node.index ? each_node_meta.index : b.id(node.index);
-	const item = node.context.type === 'Identifier' ? node.context : b.id('$$item');
+	const item = node.context?.type === 'Identifier' ? node.context : b.id('$$item');
 
 	let uses_index = each_node_meta.contains_group_binding;
 	let key_uses_index = false;
@@ -185,7 +185,7 @@ export function EachBlock(node, context) {
 	if (!context.state.analysis.runes) sequence.push(invalidate);
 	if (invalidate_store) sequence.push(invalidate_store);
 
-	if (node.context.type === 'Identifier') {
+	if (node.context?.type === 'Identifier') {
 		const binding = /** @type {Binding} */ (context.state.scope.get(node.context.name));
 
 		child_state.transform[node.context.name] = {
@@ -218,7 +218,7 @@ export function EachBlock(node, context) {
 		};
 
 		delete key_state.transform[node.context.name];
-	} else {
+	} else if (node.context) {
 		const unwrapped = (flags & EACH_ITEM_REACTIVE) !== 0 ? b.call('$.get', item) : item;
 
 		for (const path of extract_paths(node.context)) {
@@ -260,11 +260,12 @@ export function EachBlock(node, context) {
 	let key_function = b.id('$.index');
 
 	if (node.metadata.keyed) {
+		const pattern = /** @type {Pattern} */ (node.context); // can only be keyed when a context is provided
 		const expression = /** @type {Expression} */ (
 			context.visit(/** @type {Expression} */ (node.key), key_state)
 		);
 
-		key_function = b.arrow(key_uses_index ? [node.context, index] : [node.context], expression);
+		key_function = b.arrow(key_uses_index ? [pattern, index] : [pattern], expression);
 	}
 
 	if (node.index && each_node_meta.contains_group_binding) {

packages/svelte/src/compiler/phases/3-transform/server/visitors/EachBlock.js

@@ -21,7 +21,11 @@ export function EachBlock(node, context) {
 	state.init.push(b.const(array_id, b.call('$.ensure_array_like', collection)));
 
 	/** @type {Statement[]} */
-	const each = [b.let(/** @type {Pattern} */ (node.context), b.member(array_id, index, true))];
+	const each = [];
+
+	if (node.context) {
+		each.push(b.let(node.context, b.member(array_id, index, true)));
+	}
 
 	if (index.name !== node.index && node.index != null) {
 		each.push(b.let(node.index, index));

packages/svelte/src/compiler/phases/scope.js

@@ -527,6 +527,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 			const scope = state.scope.child();
 			scopes.set(node, scope);
 
+			if (node.context) {
 				// declarations
 				for (const id of extract_identifiers(node.context)) {
 					const binding = scope.declare(id, 'each', 'const');
@@ -552,6 +553,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 
 				// Visit to pick up references from default initializers
 				visit(node.context, { scope });
+			}
 
 			if (node.index) {
 				const is_keyed =

packages/svelte/src/compiler/types/template.d.ts

@@ -401,7 +401,8 @@ export namespace AST {
 	export interface EachBlock extends BaseNode {
 		type: 'EachBlock';
 		expression: Expression;
-		context: Pattern;
+		/** The `entry` in `{#each item as entry}`. `null` if `as` part is omitted */
+		context: Pattern | null;
 		body: Fragment;
 		fallback?: Fragment;
 		index?: string;

packages/svelte/tests/runtime-runes/samples/each-without-as/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<div>hi</div> <div>hi</div> <div>0</div> <div>1</div>`
+});

packages/svelte/tests/runtime-runes/samples/each-without-as/main.svelte

@@ -0,0 +1,7 @@
+{#each [10, 20]}
+	<div>hi</div>
+{/each}
+
+{#each [10, 20], i}
+	 <div>{i}</div>
+{/each}

packages/svelte/types/index.d.ts

@@ -1193,7 +1193,8 @@ declare module 'svelte/compiler' {
 		export interface EachBlock extends BaseNode {
 			type: 'EachBlock';
 			expression: Expression;
-			context: Pattern;
+			/** The `entry` in `{#each item as entry}`. `null` if `as` part is omitted */
+			context: Pattern | null;
 			body: Fragment;
 			fallback?: Fragment;
 			index?: string;